400
UNIT 3
Organ Systems
Tunica Vasculosa (Tunica Media)
Anterior
Posterior
Fig. 1-1C
Fig. 20-9B
Fig. 20-9C,D
Anterior chamber
Cornea
Iris
Posterior chamber
Position of
zonular fibers
Ciliary
body
Lens
A
Figure 20-9A.
Overview of the iris and nearby structures.
H&E,
3
11
A low-magnif cation photomicrograph oF the anterior part oF the eye
is shown. The
cornea
,
iris
,
lens
,
ciliary body
, and their anatomical
relationships are reviewed here. The iris arises From the anterior part
oF the ciliary body and separates the
anterior
and
posterior chambers
.
The iris covers part oF the anterior surFace oF the lens and Forms the
pupil, which regulates the amount oF light that enters the eye.
Figure 20-9B.
Iris.
H&E,
3
136
The
anterior iridial border
(
anterior surface of the iris
) consists oF a
discontinuous layer oF f
broblasts and melanocytes. Beneath it is a
thick layer oF loose connective tissue, called the
iris stroma
(
stroma
iridis
), which contains some f
bers, cells (f
broblasts and melano-
cytes), and blood vessels. The
posterior surface of the iris
is
covered
by two layers oF heavily
pigmented epithelial cells
, which com-
pletely block light coming into the eye (except the light through the
pupil). These are the
anterior iridial epithelium
(
anterior
pigment
epithelium
) and the
posterior iridial epithelium
(
posterior pigment
epithelium
). (See also ±ig. 20-8D.)
Figure 20-9C.
Constrictor pupillae muscle and its function.
H&E,
3
272; inset
3
628
The pigment epithelium oF the iris is partially in contact with the
lens capsule
. There is a thick layer oF concentrically oriented smooth
muscle in the stroma oF the iris, which is called the
constrictor
pupillae muscle
(
sphincter muscle
). It is innervated by
postgangli-
onic parasympathetic ± bers
From the
ciliary ganglion
and serves to
decrease the pupillary size when the eye is exposed to strong light.
Figure 20-9D.
Dilator pupillae muscle and its function.
H&E,
3
473; inset
3
680
The
dilator pupillae muscle
oF the iris is composed oF radially
arranged myoid processes oF myoepithelial cells oF the anterior pig-
mented epithelium, located more peripherally in the iris than the
constrictor pupillae muscle (also see ±ig. 20-8B). It is innervated by
postganglionic sympathetic ± bers
From the
superior cervical gan-
glion
and serves to increase pupillary size when the light is dim.
Anterior iridial border
Iris stroma
(stroma iridis)
Anterior
iridial
epithelium
Constrictor
pupillae
muscle
Dilator
pupillae
muscle
Lens
Posterior
iridial
epithelium
B
C
Iris stroma
Iris stroma
Constrictor
pupillae muscle
Lens
Lens
Lens capsule
Iris stroma
Lens
Anterior iridial
border
Constrictor pupillae
Constrictor pupillae
muscle
muscle
Constrictor pupillae
muscle
D
Anterior
Anterior
iridial
iridial
epithelium
epithelium
Posterior
Posterior
iridial
iridial
epithelium
epithelium
Melanocytes
Melanocytes
Melanocytes
Dilator
pupillae
muscle
Anterior
iridial
epithelium
Posterior
iridial
epithelium
Dilator pupillae
Dilator pupillae
muscle
muscle
Dilator pupillae
muscle
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